A major advance in understanding the molecular basis for obesity occurred with the cloning of the ob gene. The mouse ob gene (GenBank Accession No. U22421) and its human homolog (GenBank Accession No. NM—000230) encode an adipose tissue-derived signaling factor for body weight homeostasis (see, e.g., Zhang, et al., 1994. Nature 372: 425–432). The mouse ob gene encodes a 4.5-kilobase adipose tissue mRNA (see, e.g., Montague, et al., 1997. Nature 387: 903–908) with a widely conserved 167-amino acid open reading frame and a 21-amino acid secretory signal sequence. The predicted amino acid sequence of the protein product of this gene, leptin (from the Greek leptos, meaning thin), is 84% identical in humans and mice and has features of a secreted protein (see, e.g., Zhang, et al., 1994. Nature 372: 425). The protein product of this gene, leptin, which has been postulated to act as a blood-borne hormone responsible for weight maintenance, is a 16-kDa plasma protein synthesized and secreted by adipocytes (see e.g., Halaas, et al., 1995. Science 269: 543–546; Pelleymounter, et al., 1995. Science 269: 540–543; Weigle, et al., 1995. J. Clin. Invest. 96: 2065–2070). The ob/ob mouse phenotype has been attributed to a deficiency in active leptin. Recombinant ob protein purified from Escherichia coli can correct the obesity related phenotypes in ob/ob mice when exogenously administered (see e.g., Campfield, et al., 1995 Science 269: 546; Pellymounter, et al., 1995. Science 269: 540; Stephens, et al., 1995. Nature 377: 530). Weight-reducing effects of recombinant leptin were also observed in normal mice and mice with diet-induced obesity.
The leptin receptor (OB-R) gene has been cloned (GenBank Accession No. AF098792) and mapping to the db locus (see, e.g., Tartaglia, et al., 1995. Cell 83: 1263–1271). Several transcripts of the OB-R, resulting from alternative splicing, have also been identified. Defects in OB-R produce a syndrome in the mutant diabetic db/db mouse that is phenotypically identical to the ob/ob mouse (see, e.g., Ghilardi, et al., 1996. Proc. Natl. Acad. Sci. USA 93:6231–6235). In contrast to ob/ob mice, however, administration of recombinant leptin to C57BLKS/J-m db/db mice does not result in reduced food intake and body weight (see, e.g., Roberts and Greengerg, 1996. Nutrition Rev. 54: 41–49).
Interestingly, obese humans and rodents (other than ob/ob mice) are not defective in their ability to produce leptin mRNA or protein and generally produce higher levels than lean individuals (see e.g., Maffei, et al., 1995. Nature Med 1: 1155; Considine, 1995. J. Clin. Invest. 95: 2986; Hamilton, et al., 1995. Nature Med. 1: 953). This data suggest that resistance to normal or elevated levels of leptin may be important factors in human obesity. Recent findings suggest that administration of recombinant leptin, or leptin mimetics of even higher potency than leptin, may be possible approaches to the treatment of at least some forms of human obesity.
Leptin is the afferent signal in a negative feedback loop regulating food intake and body weight. The leptin receptor is a member of the cytokine receptor family. Leptin's anorexigenic effect is dependent on binding to homodimer of the Ob-Rb isoform of this receptor which encodes a long intracytoplasmic domain that includes several motifs for protein-protein interaction. Ob-Rb is highly expressed in the hypothalamus suggesting that this brain region is an important site of leptin action. Mutation of the mouse ob gene has been demonstrated to result in a syndrome that exhibits pathophysiology that includes: obesity, increased body fat deposition, hyperglycemia, hyperinsulinemia, hypothermia, and impaired thyroid and reproductive functions in both male and female homozygous ob/ob obese mice (see e.g., Ingalis, et al., 1950. J Hered 41: 317–318. Therapeutic uses for leptin or leptin receptor include (i) diabetes (see, e.g., PCT Patent Applications WO98/55139, WO98/12224, and WO97/02004); (ii) hematopoiesis (see, e.g., PCT Patent Applications WO97/27286 and WO98/18486); (iii) infertility (see, e.g., PCT Patent Applications WO97/15322 and WO98/36763); and (iv) tumor suppression (see, e.g., PCT Patent Applications WO98/48831), each of which are incorporated herein by reference in their entirety.
Numerous, prior clinical trials that have attempted to treat obese individuals with recombinant leptin have met with markedly limited success. Typical serum leptin concentrations in most obese humans are elevated above those of non-obese individuals by as much as 5-fold. However, the cerebral spinal fluid (CSF) of these obese individuals often have normal leptin levels, thus suggesting that the rate-limiting factor contributing to leptin resistance in these individuals may be related to defective transport of leptin through the Blood-Brain Barrier and into the Central Nervous System (CNS).
The mature form of circulating leptin is a 146-amino acid protein that is normally excluded from the CNS by the blood-brain barrier (BBB) and the blood-CSF barrier. See, e.g., Weigle et al., 1995. J Clin Invest 96: 2065–2070. Most leptin-related studies able to report weight loss activity from administration of recombinant leptin, leptin fragments and/or leptin receptor variant have administered said constructs directly into the ventricles of the brain. See e.g., Weigle, et al., 1995. J Clin Invest 96: 2065–2070; Barash, et al., 1996. Endocrinology 137: 3144–3147. Administration of any treatment directly into the brain has serious drawbacks for widespread use of such treatment in the human population. Only studies by the inventors have been able to show significant weight loss activity due to administered of leptin peptides by more favorable methods, namely, through intraperitoneally (i.p.) administration, to test subjects. See, Grasso et al., 1997. Endocrinology 138: 1413–1418.
Leptin fragments, and most particularly an 18 amino acid fragment comprising residues 57VTGLDFIPGLHPILTLSK74 (SEQ ID NO:19) taken from full length human leptin (the full length sequence is shown in SEQ ID NO:17), have been reported to function in weight loss, but only upon direct administration through an implanted cannula to the lateral brain ventricle of rats. See, e.g., PCT Patent Applications WO97/46585, which is incorporated herein by reference in its entirety. Those fragments in PCT Patent Applications WO97/46585 are different from the fragments of this invention.
There is some evidence that leptin enters the brain via a saturable transport system. See, e.g., Banks et al., 1996, Peptides, 17: 305–311. Because the majority of obese humans do not have elevated cerebrospinal fluid (CSF) levels of leptin, even though their plasma levels may be five-fold higher when compared to nonobese individuals, the rate-limiting factor associated with leptin resistance in human obesity may be related to defective leptin transport into the central nervous system (CNS). See, e.g., Caro et al., 1996. Lancet 348: 159–161; Schwartz et al., 1996. Nat Med 2: 589–593. The ability of centrally administered leptin to reduce food intake and body weight gain in diet-induced obese mice resistant to peripherally administered leptin, is consistent with a mechanism of obesity which may involve saturated or defective leptin transport. See, e.g., Van Heek et al., 1997. J Clin Invest 99: 385–390.
Thus, efforts to develop leptin-related peptide agonists of low molecular weight, or nonpeptide mimetics that can be transported across the BBB and blood-CSF barrier by mechanisms independent from those by which leptin is transported take on added importance. Identification of active epitopes within the leptin molecule, therefore, is important to the development of leptin analogs which can be administered peripherally, and thus have potential usefulness in the treatment of human obesity and its related dysfunctions.
There is a current need for methods and related compositions for use in detecting physiological obesity or other conditions related to abnormalities of the endogenous leptin pathway. There remains an as yet unfulfilled need for the development of low molecular weight, highly-potent peptide agonists of leptin, or nonpeptide leptin mimetics which are permeable to the Blood-Brain Barrier and can thus enter the central nervous system (CNS) without assisted-transport. The development of such pharmacophores may ultimately lead to novel methods of treatment for physiological obesity and/or other conditions which are related to abnormalities of endogenous leptin pathway, as well as a possible extension of their application to other obesity-related dysfunctions (e.g., Type II or non-insulin-dependent diabetes mellitus (NIDDM)).
The citation of any reference herein should not be deemed as an admission that such reference is available as prior art to the instant invention.